The present invention relates generally to laser diodes and fiber optics and more particularly to a technique for efficiently coupling the output of a laser diode to an optical fiber.
Laser diodes have emerged as important new sources of optical energy. Their small size and high electrical to optical conversion efficiencies make them attractive alternatives to more conventional lasers such as ion and solid state lasers in many commercial applications, including medicine and industry. One example of a medical application is directing the laser's infrared radiation (referred to as the light beam or simply the light) into one end of an optical fiber inserted into an eye, and causing the light emerging from the other end of the fiber to illuminate a small area on the retina.
Laser diodes are not without their drawbacks, however, a most notable one being the difficulty of coupling their output into an optical fiber. The laser output is highly asymmetric, with an elliptical beam profile. Additionally, the numerical aperture (n.a.), defined as the sine of the half angle of the diverging fan of light multiplied by the index of the medium in which the angle is being measured (typically air with an index n of 1.000), differs by a factor of 3 to 5 between the major and minor axes of the beam. These profiles are created at least in part by a semiconductor laser gain medium whose width to height ratio may be between 4 and 10,000, depending on specific diode type, and an active region that allows waveguided modes to propagate in one axis and free space modes in the orthogonal axis. Additionally, the laser diode is an astigmatic source. That is, the fans of rays in the planes of the major and minor axes do not diverge from the same point.
Resultant laser beams from such devices with power outputs in the 100 to 5000 milliwatt range often are nearly diffraction limited in one axis (the short aperture dimension, perpendicular to the diode junction) and many times diffraction limited in the orthogonal axis (the long aperture dimension, parallel to the diode junction). With these beam characteristics, it is not hard to appreciate the difficulties encountered when trying to efficiently deliver these beams to circular fiber optics.
One coupling technique is to permanently fasten the emitting facet of the laser to the face of a fiber having a core diameter equal to or greater than the larger dimension of the emitting aperture. This intrinsically produces an output beam of reduced optical brightness, as the waveguiding quality of the fiber yields a radially symmetric output beam with n.a. equal to the largest input n.a. and beam diameter equal to the fiber diameter (itself already equal to or greater than the largest linear dimension of the laser diode aperture). Attempts to reduce output beam dimensions by using undersize optical fibers or to minimize output n.a. by using fibers manufactured with low n.a. characteristics themselves invariably result in a decrease in coupling efficiency. Moreover, the permanence of the connection leaves no convenient way to disconnect or exchange fibers, i.e. for cleaning, repair, or sterilization.
A second technique is to image the emitting aperture on the entrance face of the fiber. A first, typically high n.a., lens efficiently collects and collimates the laser output. A second lens focuses the beam onto the face of the fiber core. As this technique employs an image of the emitting aperture and not the aperture itself, it is somewhat easier to incorporate removable fiber optics. Transmission through the fiber again results in a radially symmetric beam with output n.a. not less than the largest input n.a. and output diameter not less than the largest linear dimension of the input beam image. As described previously, attempts to implement fibers with smaller diameters or lower n.a. can only result in poorer coupling efficiencies: the intrinsic brightness of an optical source cannot be increased by reimaging, or indeed by any optical technique.